In a boiling water nuclear reactor, a plurality of nuclear fuel bundles are provided, each including a matrix of upstanding fuel rods. Each fuel rod contains a substantial number of stacked, cylindrical pellets of enriched uranium, the fuel rods being sealed at opposite ends. The fuel rods of each bundle are supported on a lower tie plate and extend upwardly from the lower tie plate to an upper tie plate. The fuel bundle is thus defined by a lattice of rows and columns of vertically extending, generally parallel, laterally spaced fuel rods. In addition to supporting the fuel rods, the lower tie plate permits entry of coolant/moderator water within the fuel bundle. The upper tie plate maintains the matrix of fuel rods upright from the lower tie plate and permits the discharge of heated water and generated steam from the interior of the fuel bundle. The fuel bundle also includes spacers, for example, seven such spacers, spaced along the length of the fuel bundle. The spacers serve to maintain the fuel rods laterally spaced from one another and are disposed at intervals along the lengths of the fuel rods. A channel surrounds each fuel bundle and extends from the lower tie plate to the upper tie plate. The channel confines the water flow from the lower tie plate to the upper tie plate. The fuel bundles are positioned in the reactor core between a lower core plate and an overlying top guide.
In operation, water enters each fuel bundle through its lower tie plate. The water acts both as a coolant and a moderator. As a coolant, the water is heated and evolves to steam by heat exchange with the fuel rods. As a moderator, fast neutrons from the nuclear reaction are moderated and become slow-moving or thermal neutrons to continue the chain reaction within the fuel bundle. As the water is heated, the bundles have increasing fractions of steam in the upper two-phase portion of the fuel bundle.
It has been found beneficial to provide a plurality of less than full-length fuel rods in the nuclear fuel bundle and these rods are called part-length fuel rods. These part-length fuel rods are supported on a lower tie plate and extend upwardly toward the upper tie plate, terminating short of the upper tie plate in or just above one of the upper spacers, for example, the fifth or sixth spacer. Thus, a vent volume is defined between the upper tie plate and the upper ends of the part-length fuel rods. It is known to provide in this vent volume a steam vent tube and liquid deflector. See, for example, U.S. Pat. No. 5,091,146, of common assignee herewith. The steam vent tube and liquid deflector of that patent was not provided with capability for removal to provide access to the underlying pad-length fuel rods.